Myelodysplastic syndromes (MDS) belong to the most frequent bone marrow diseases with a crude incidence of about 4 in 100,000 per year. The diagnosis of MDS still is mainly based on morphologic findings in blood and marrow. The new WHO classification system takes into account the medullary and peripheral blast count as well as the degree of dysplasia in the different cell lines. To correctly identify MDS types, cytogenetic evaluation is of importance, as the WHO classification introduced the entity MDS with del(5q), which is characterized by special morphologic and hematologic features. The separation of MDS from acute leukemias has been redefined using a cutoff value of 20% peripheral and/or medullary blasts. The International Prognostic Scoring System still is the gold standard in prognostication, but new items like transfusion need will be used more and more and have been incorporated into the WHO adapted Prognostic Scoring System. In childhood, MDS is uncommon, accounting for less than 5% of all hematopoietic neoplasms in patients less than 14 years of age. To accommodate for the characteristics of pediatric MDS, a simple classification scheme based on morphological features and conforming with the WHO suggestions was proposed. The dysplastic prodrome of acute myeloid leukemia in Down syndrome is classified within myeloid leukemia in Down syndrome and excluded from the population-based studies of MDS.